Effective Offset with Spacer Calculator
Calculate the effective wheel offset after adding a hub spacer. Subtract spacer thickness from ET for 3 mm to 25 mm spacers.
Account for spacer thickness.
Effective Offset with Spacer Calculator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Effective Offset with Spacer Calculator Works?
The Effective Offset with Spacer Calculator works by subtracting the spacer thickness from the wheel's stamped offset. A spacer moves the mounting face outward by its full thickness. The tool uses 3 steps.
Enter the wheel offset
Type the stamped ET, for example ET45.
Enter spacer thickness
Type the spacer thickness in mm or inches, for example 15 mm (0.59 in).
Read the effective offset
The calculator returns ET30, the offset the wheel now behaves like.
Effective Offset with Spacer Calculator Formula
The Effective Offset with Spacer Calculator formula is ET_effective = ET_wheel − Spacer thickness.
ET_eff = ET − t_sBS_eff = BS − t_st_s(mm) = t_s(in) × 25.4- ET
- Stamped wheel offset (mm)
- t_s
- Spacer thickness (mm)
- ET_eff
- Offset the wheel behaves like (mm)
Worked example
An ET50 wheel with a 1 in (25.4 mm) spacer behaves like ET24.6. A 5 mm (0.20 in) spacer on an ET35 wheel gives ET30.
Effective Offset with Spacer Calculator Conversion Table
The Effective Offset with Spacer Calculator conversion table lists effective offset for 4 wheel offsets and 6 spacer thicknesses from 3 mm (0.12 in) to 25 mm (0.98 in).
| Wheel ET | 3 mm | 5 mm | 10 mm | 15 mm | 20 mm | 25 mm |
|---|---|---|---|---|---|---|
| ET50 | ET47 | ET45 | ET40 | ET35 | ET30 | ET25 |
| ET45 | ET42 | ET40 | ET35 | ET30 | ET25 | ET20 |
| ET40 | ET37 | ET35 | ET30 | ET25 | ET20 | ET15 |
| ET35 | ET32 | ET30 | ET25 | ET20 | ET15 | ET10 |
What is Effective Offset with Spacer Calculator?
An Effective Offset with Spacer Calculator is a tool that converts a wheel offset plus a spacer into a single equivalent offset. Retailers list wheel fitment by offset, so the equivalent value lets an owner compare a spacer setup with a wheel of lower offset.
Spacers thicker than about 5 mm (0.2 in) on a bolt-on design reduce thread engagement. Thicker spacers use their own studs (bolt-on adapters) or need longer studs or bolts.
Hub-centric spacers keep the wheel centered on the hub. Lug-centric spacers depend on the lug seats for centering.
What are the uses of Effective Offset with Spacer Calculator?
There are 4 main uses of the Effective Offset with Spacer Calculator, including fitment matching and brake clearance.
Fitment matching
Match a forum-recommended offset with a spacer.
Brake clearance
Move a wheel off a large caliper face.
Strut clearance
Push a wheel out to clear the strut.
Testing before buying
Try an offset with spacers before ordering new wheels.
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